3DEM
is another program that the serious terrain modeler needs to have in their
library of GIS applications. You can download a the latest (free)
version of 3DEM
to experiment with. You can also get
an older version of the software called 3DEM70
for free. The older version is kind of
crippled because it will not read the revised SDTS file format. (The USGS,
working through GeoCommunity for reasons that have to do with file compression
made minor modification to the xxxx.cel0.ddf file format in the
SDTS profile. While the change was very much within the existing SDTS file
specification, it immediately caused about half the SDTS readers in the world to
become obsolete pending revision. Since no one is maintaining older
software like 3DEM70, users of these programs are out of luck.) You can
still get your files into 3DEM70 by using the utility SDTS2DEM
to convert SDTS to USGS DEM format.

The best thing about 3DEM, and the reason that you need it is for its ability to
overlay USGS topo maps over DEMS. Recall in my section above on this
technique I outlined how to do this using POVRay and Paintshop Pro. The
way described above is the hard way. The great thing about 3DEM is its
automatic registration of USGS DRGs to the corresponding USGS DEM. In
particular, the overlay of 1:24,000 topos over 1:24000 SDTS 7.5' 30m DEMS is
pretty seamless and is the technique that I will demonstrate in this tutorial.
So here is how to do it:

The first thing that you need to do is go to gisdatadepot.com
and download the 7.5' 30m SDTS DEM of your
area of interest. You do this by clicking on a state, then a county, then
on 'Digital Elevation Models, 24K (for 1:24,000), and then on your quad.
Download the file. The USGS topo can be downloaded from a couple of free websites. Try www.topowest.com for starters. If your state is not offered at this site, see the information in the
News section for more information.

Then back up one page and click on 'Digital Raster Graphics (DRG), 24K. Go
through the same exercise and select the corresponding USGS topo quad that
matches up with the DEM you just selected. Beware that these files are
pretty big ( several megabytes) and will get huge (~40MB) when unzipped.

After you have both files downloaded and unzipped, open 3DEM. Click
<File>, <Digital Model>, <Terrain Model>. Make sure that 'USGS SDTS DEM'
is selected and load the DEM file. You should see a familiar color depth
DEM image. Select <Operation>, <Apply Overlays> under the
'Terrain' window. Select your topo map file and click <Load>. 3DEM
will load the topo. (This step will take a few minutes as it processes the file). Once the
file is loaded, select <Crop>. A crop window will come up. Adjust the crop box to trim the
map borders and press <Enter>. After a few minutes, 3DEM will crop the border and
register and overlay the topo map over the DEM.

At this point, all you have to do is set the 3DEM viewing parameters in
order to view your overlay. I redid the Salem, NY overlay that I described
in the previous section, using the 3DEM utilities. In fairness, the
stunning qualify of the 3DEM overlay is directly related to the high quality of
the DRG used in this tutorial. However, making this overlay was a snap
compared to the agony of doing it in Painshop.

3DEM also has utilities for manual registration of DRGs. The process is
obviously more involved than that procedure described here. However, it
gives 3DEM great flexibility in overlaying unmatched DRG data over DEMS.